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Why galaxies spin?

  1. Nov 3, 2013 #1
    I wondered about them sometime; I came across many theories, such as it might be because the of the black hole at its centre, but another question rises!! Why black holes spin?
    I thought it might be because at first when the black hole was a star just like our sun, before supernovae happening, at its first moments that matter collided together, gravity made them turn around each other then eventually collision and then birth of a star!!
    Any suggestions?
     
  2. jcsd
  3. Nov 3, 2013 #2

    phinds

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    Planets, black holes, galaxies all spin for the same reason. The matter that forms them does not come together all headed in directly to a single point. Also, the stars that form black holes are rotating before they go supernova. Why would you expect that rotational inertia to just disappear?
     
  4. Nov 3, 2013 #3
    By my statement, I meant before the creation of even stars, matter collided in a circular direction because of their gravity, then star was made, supernovae and then black holes, both bodies spin but I meant rotation of black holes or planets may accrued way back when their first parts was joined to make them.
     
  5. Nov 3, 2013 #4

    phinds

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    Yes, that's exactly what I just said.
     
  6. Nov 4, 2013 #5

    Chalnoth

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    Another way to state it is that it is really, really unlikely for a large, randomly-moving cloud of particles to have no angular momentum at all. It likely won't have enough to have any noticeable spin while it's still a diffuse cloud of gas. When that cloud of particles collapses over time to form a compact structure like a galaxy, the angular momentum stays close to the same, but because a galaxy (or a star, or a planet, or a black hole) is vastly, vastly more compact than the cloud of dust that it forms from, it spins noticeably.

    And for spiral galaxies, the reason why the stars mostly tend to orbit in a plane is because those galaxies are full of dust, and thus the stars experience friction. Galaxies with little to no dust form elliptical galaxies, which have stars orbiting in essentially random directions.
     
  7. Nov 4, 2013 #6
    So what you are saying is that clouds of dust just start to spin after being compressed down?
     
  8. Nov 4, 2013 #7

    Chalnoth

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    Sorta kinda. More that large clouds of gas are already spinning on average by some imperceptible amount just due to the random motions of the particles in the cloud. You'd never notice it for a large cloud.

    But as the cloud collapses in on itself due to gravity, that spinning becomes very noticeable indeed.
     
  9. Nov 4, 2013 #8
    Thanks, now I got more clear view of it.
     
  10. Nov 4, 2013 #9

    Nugatory

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    Well, that is how angular momentum works...

    It's not impossible to find a non-spinning galaxy, but it would be surprising, for about the same reason that it would be surprising to throw a coin and have it land balanced on its edge. There's no violation of the laws of physics, but the initial conditions that would lead to that result are unlikely. Likewise, it's very unlikely that you'll find a cloud of gas that has zero angular momentum but if you did, it could collapse down to a non-rotating galaxy.
     
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